Where scrollbars are clicked, and why

Scrolling is a widely used mean to interact with visual displays, usually to move content to a certain target location on the display. Understanding how user scroll might identify potentially suboptimal use and allows to infer users’ intentions. In the present study, we examined where users click on a scrollbar depending on the intended scrolling action. In two online experiments, click positions were systematically adapted to the intended scrolling action. Click position selection could not be explained as strict optimization of the distance traveled with the cursor, memory load, or motor-cognitive factors. By contrast, for identical scrolling actions click positions strongly depended on the context and on previous scrolls. The behavior of our participants closely resembled behavior observed for manipulation of other physical devices and suggested a simple heuristic of movement planning. The results have implications for modeling human–computer interaction and may contribute to predicting user behavior. Supplementary Information The online version contains supplementary material available at 10.1186/s41235-024-00551-z.

Significant effects are marked green.

Effect of Input Device
Table ESM-2 shows the results of an ANOVA on the initial vertical cursor position with within-participant factors of target number (1,2,3,4,6,7,8,9), vertical start position (low, center, high), and between-participant factor input device (touchpad, mouse).The participants (n = 3) who responded to have used both input devices were not included in the ANOVA.The effect of the input device manifested in the three-way interaction.Descriptively, touchpad users clicked closer to the screen center than mouse users.Experiment 2

Effect of Input Device
Table ESM-3 shows the results of an ANOVA on the initial vertical cursor position with within-participant factors of target number (13, 17), inducer eccentricity (low, medium, high), and between-participant factor input device (touchpad, mouse).The participant (n = 1) who responded to have used both input devices was not included in the ANOVA.The effect of the input device manifested in the four-way interaction.Figure ESM-3 shows that the interaction of target number and eccentricity increased over subblocks for mouse users but not for touchpad users.

Effect of test trial repetitions
To test whether the length of a sequence of test trials affects initial vertical cursor positions, we split the test trial data of Experiment 2 by participant, target number (13, 17), inducer eccentricity (low, high), and the numbers of directly preceding test trials (i.e.repetition: 0, 1, 2 or more).Table ESM-4 shows the results of the ANOVA on the 44 participants that delivered data for all cells.As the critical three-way interaction almost reached significance, we computed contrasts for this interaction for 0 versus 1 repetition and 1 versus 2 or more repetitions.Test trials were more similar when preceded by a test trial than when not, t(1,43) = 2.60, p = .013,dz = 0.39.However, this effect did not increase when comparing test trials preceded by one versus two or more test trials, t(1,43) = 0.16, p = .874,dz = 0.02.Finally, ANOVAs with factors of target number and inducer eccentricity conducted separately for each level of repetition all revealed significant interactions, all F(1,43) ³ 29.45 , all ps £ .001,all ges ³ .11.Significant effects are marked green.

Figure ESM-5 Initial vertical cursor position by target number, eccentricity, and repetition in Exp. 1
Note: Error bars show 1 SEM.

Interindividual differences
In the following, we consider whether participants can be split into subgroups that apply different strategies.To simplify the analysis and allow the comparison between experiments, we fitted functions to the initial vertical click positions and compare the resulting parameters.More specifically, we first simplified the data.First, we removed general vertical biases by centering the click positions of each participant on zero.Then, the sign of click positions for downward scrolls was reversed.Next, we computed linear regressions for each participant of both experiments (independent variable: absolute extent of the required scroll in screen coordinate; dependent variable: (mirrored) click position.The slope of the linear regression indicates how strongly the extent of the scroll affects click positions.The intercept the effect of the direction per se.We refer to both parameters as effect of extend and effect of direction, respectively.As an example, consider panel D of Figure ESM-6.The red line shows the model, and the dots represent the participant's mean click positions.The effect of extent corresponds to the slopes of both lines.The effect of direction reflects 50% of the vertical offset between both lines.
Figure ESM-6A shows scatter plots for the effect of direction and extent.Figure ESM-6B shows data of exemplary participants to give an impression of the meaning of the parameters.The figures also shows that the linear model fits the data.The inspection of the figure leads to several conclusions.First, the effect of extent assumes positive values smaller than one.This reflects that increasing the extent of a required scroll by some value alters the initial vertical click positions only by a fraction of that value.Second, the typically positive effect of direction indicates that the movement direction per se appears to have an additional effect on click position.Third, participants differ but cannot be divided into obvious subgroups.An exception may be the five or six participants in the lower left quadrant of Experiment 2, who only slightly, if at all, adapted the click position to the task.Third, while the input device may have a statistical influence on participants behavior, the scatterplots show considerable overlap between mouse users and touchpad users.There is a negative correlation of the effects of direction and extent, r = .40,t(79) = -3.85,p < .001.This reflects that due to the limits of the Low (14,16) screen size, both effects cannot play out in full at the same time.In summary, the data show that participants differ in how they adapt click positions to an upcoming task.However, these differences do not appear to be the result of qualitatively different strategies but rather appear to reflect a continuum.

Figure ESM-6
Parameters of models fitted to each participant's initial vertical click positions.
Note: The data of the marked participants in A are shown in more detail in B. In B, the black dots reflect empirical data whereas the red lines reflect the fit of the model.The effect of extent corresponds to the slope of the red lines.The effect of directions corresponds to 50% of the vertical offset between the lines for upward and downward scrolls.
In addition, we checked whether click position selections, the average time needed to complete the scrolling action (scroll time: time from the first click on the scroll bar to the end of the trial), and the average number of submovements, depended on the time participants took to initiate the movement toward the scroll bar (approach reaction time: time from click on start square until the cursor was displaced at least five pixel from the position it had at the time of the click), the participant's age, gender, or input device.The effect of continuous variables was analyzed with correlations.The effect of discrete variables was analyzed with Welsh tests.ESM Tables ESM-5 to ESM-8 show the results.The data reveal that participants that initiate the movement toward the number line slower select click positions stronger on movement direction per se.These participants are also slower in completing the scrolling action, suggesting an overall lower performance, rather than a speed-accuracy trade-off concerning the click position selection.Neither age nor sex predicts click selections, the duration of the scrolling action, or the number of submovements.Finally, as revealed in previous analysis, mouse user's click positions depend more linearly and stronger on the target number (see also Figure ESM-1) and they complete the task considerably faster.

Figure ESM- 1
Figure ESM-1 Initial vertical cursor position by input device, target number, and start position in Exp. 1

Figure ESM- 2
Figure ESM-2 Initial vertical cursor positions by target number, start position, and participant for Exp. 1

Figure ESM- 4
Figure ESM-4 Initial vertical cursor position in test trials by target number, inducer eccentricity, and participant for Exp. 2